Analysis of Hydrogen Atom Transfer in Photoexcited Indole(NH₃)_n Clusters by Femtosecond Time-Resolved Photoelectron Spectroscopy^†

The photoinduced H-atom-transfer reaction in indole(NH₃)_n clusters has been analyzed by femtosecond time-resolved photoelectron−photoion coincidence spectroscopy. The different contributions to the measured time-dependent ion and electron signals resulting from ionization by one and two probe photons can be discriminated and analyzed separately. In particular, the distinctively different dynamical behavior observed for clusters with small (n = 1−3) and larger (n ≥ 4) numbers of ammonia molecules is elucidated. For the small clusters an ultrafast process with a time constant of about 150 fs is identified and attributed to internal conversion from the initially excited ππ* state to the πσ* state. In contrast, for the larger clusters (n ≥ 4) such an initial ultrafast process is not observable probably for Franck−Condon reasons, while a structural rearrangement mechanism after the H transfer on a time scale of 10 ps is clearly recognized.